Nowadays, industrial facilities and structures are demanded to meet the needs for durability, life extension, and a maintenance free property, and stainless steel is receiving a lot of attention as a material meeting these demands. On the other hand, alloy elements typified by Ni, Mo, and Cr which are main raw materials of stainless steel tend to undergo increases and changes in price. Therefore, recently, instead of simple stainless steel, stainless clad steel has received a lot of attention as a steel material with which it is possible to utilize the excellent corrosion resistance of stainless steel and which is economical due to a stable and low price. “Stainless clad steel” means a steel material manufactured by bonding two kinds of metal materials having different properties, in which stainless steel is used as a cladding material and plain steel is used as a base material. Since clad steel is manufactured by metallurgically bonding different kinds of metal, there is no concern that separation may occur, which may occur in coated steel, and it is possible to provide new properties which are not achieved by using a single substance of metal or alloy.
To achieve corrosion resistance suitable for the usage environment, the kind of stainless steel used as a cladding material is selected in accordance with the environment in which the stainless clad steel is used, and corrosion resistance equivalent to that of simple stainless steel (consisting of only stainless steel throughout the thickness) is achieved. By using stainless clad steel in this manner, corrosion resistance equivalent to that of simple stainless steel (consisting of only stainless steel throughout the thickness) can be achieved with a small amount of stainless steel material used, and there is an advantage that both cost efficiency and functionality are achieved.
As described above, stainless clad steel is a very advantageous functional steel material and, recently, there has been an increasing need for such a steel material in various industrial fields.
On the other hand, the passivation film of stainless steel tends to be damaged by chloride ions, and its corrosion takes the form of pitting corrosion or crevice corrosion. Therefore, seawater pitting resistance is an important index in seawater because pitting corrosion becomes a source of local corrosion, while corrosion in an acid typified by sulfuric acid and hydrofluoric acid takes the form of overall corrosion. Therefore, when stainless clad steel is used for marine structures typified by, for example, seawater desalination facilities and ship building (FPSO: Floating Production, Storage and Offloading system) or when stainless clad steel is used in an environment in which stainless clad steel is exposed to seawater, seawater pitting resistance which is sufficiently strong against a severe seawater corrosion environment is required. In addition, abrasion resistance which is sufficiently strong against impacts caused by floating objects such as ice and driftwood is also required.
As an example of a technique that increases seawater pitting resistance, Japanese Patent No. 4179133 discloses a method of manufacturing a stainless clad steel pipe, in which stainless steel with excellent seawater-resistance is used as a cladding material and carbon steel is used as a base material. It also discloses a technique of specifying conditions regarding a solution heat treatment and the chemical composition of carbon steel which is used as a base metal. However, in Japanese Patent No. 4179133, it is necessary that stainless steel used as a cladding material be selected in accordance with a use application (for example, marine structures) to achieve the corrosion resistance and seawater pitting resistance required in accordance with the use application of the stainless clad steel pipe. That is to say, only a technique in which the chemical composition of stainless steel is controlled is disclosed and, therefore, in the case of stainless clad steel, it is difficult to increase the reliability of the soundness of a bonding interface (bonding capability) and to maintain the qualities (corrosion resistance and mechanical properties) of the base material and the cladding material at the same time for all of the high-class steel materials and various grades of steel.
Japanese Patent No. 3336820 discloses a technique for providing a seawater corrosion resistant austenitic stainless cast steel and a seawater pump with excellent seawater pitting resistance. Specifically, regarding austenitic stainless cast steel having a chemical composition containing, by mass %, C: 0.08 wt % or less, Si: 0.5 to 1.5 wt %, Mn: 0.5 to 2 wt %, P: 0.04 wt % or less, S: 0.01 wt % or less, Ni: 8.0 to 9.5 wt %, and Cr: 18 to 21 wt %, the technique is realized by the stainless cast steel having a microstructure including 6 vol % or more of a 6 ferrite phase or a cleanliness of 0.1% or less. However, Japanese Patent No. 3336820 specifies only certain ranges of the chemical composition of steel and the amount of a second phase precipitated, but does not sufficiently disclose a technique regarding the surface quality and abrasion resistance of a steel plate. In addition, since the material is cast steel, there is a disadvantage in terms of, for example, strength, and the material as it is cannot be used as a raw material for clad steel.
The corrosion resistance of stainless clad steel is determined by that of stainless steel used as a cladding material. To increase the corrosion resistance of stainless steel, alloy design is used in which a pitting index which is an index representing pitting resistance is increased by increasing the amounts of rare alloy elements such as Cr and Mo added.
In addition, in the case of clad steel, since it is necessary to achieve the strength and toughness for a base material and the corrosion resistance for a cladding material at the same time, it is necessary to select manufacturing conditions different from those for simple stainless steel even if the cladding material has the same chemical composition. Therefore, there is a problem in that it is not possible to completely prevent precipitation of a σ phase which causes a serious decrease in corrosion resistance due to sensitization. Therefore, it is necessary that the area ratio of a σ phase precipitated in steel be as small as possible within a realizable range in a practical manufacturing process.
However, in the case of conventional techniques, it is necessary to increase the amounts of expensive rare alloy elements such as Cr and Mo added to increase the pitting index, and it is necessary to perform advanced heat treatments such as rapid heating and rapid cooling to decrease the area ratio of a σ-phase and, therefore, there is an increase in load on the manufacturing process, which results in a decrease in manufacturability. These all cause an increase in the manufacturing cost of cladding material.
In addition, in the case of a clad steel plate, compatibility (such as bonding capability) between a cladding material and a base material is very important in a practical manufacturing process. Regarding this, in the case where the pitting index, that is, corrosion resistance is controlled by controlling additive chemical elements, it is unavoidable to increase the amount of alloy elements added and, as a result, there is only a limited choice of the kind of a cladding material that meets the desired corrosion resistance and there is a decrease in the degree of freedom in the choice of the base material. When the degree of freedom in the choice of the base material is small, it is difficult to decrease cost and there are situations where it is not possible to manufacture a clad steel plate depending on the desired corrosion resistance.
In view of the situation described above, it could be helpful to provide seawater-resistant stainless clad steel with excellent abrasion resistance and seawater pitting resistance.